The present invention relates to proppant materials and other uses for the proppants. The present invention further relates to methods to make a proppant.
Previously, some proppants were made by Oxane Materials, Inc., so that the proppant had a template, such as a template sphere, with a shell wherein the template sphere or core typically provided a lightweight or low density material that had a void or voids, such as a cenosphere. The shell that was present preferably was a strength-bearing shell that lead to the overall crush resistance of the proppant once the shell encapsulated or coated the template sphere. This proppant holds much promise for addressing many of the disadvantages with previous proppant materials. However, if a proppant could be prepared having the same or similar lightweight qualities, as well as strength-bearing qualities, as the core/shell proppant, but without the need to form a shell on the template sphere, this would have numerous advantages with respect to costs, the amount of material used, and the like. Through a reaction of the proppant, such as on the surface, a separate shell is not needed, since the reaction forms a phase that can replace the need of a shell.
Ceramic proppants are widely used as propping agents to maintain permeability in oil and gas formations. Conventional proppants offered for sale exhibit exceptional crush strength but also extreme density. Typical densities of ceramic proppants exceed 100 pounds per cubic foot. Proppants are materials pumped into oil or gas wells at extreme pressure in a carrier solution (typically brine) during the fracturing process. Once the pumping-induced pressure is removed, proppants “prop” open fractures in the rock formation and thus preclude the fracture from closing. As a result, the amount of formation surface area exposed to the well bore is increased, enhancing recovery rates. Proppants also add mechanical strength to the formation and thus help maintain flow rates over time. Three grades of proppants are typically employed: sand, resin-coated sand and ceramic proppants. Proppants are principally used in gas wells, but do find application in oil wells.
Relevant quality parameters include: particle density (low density is desirable), crush strength and hardness, particle size (value depends on formation type), particle size distribution (tight distributions are desirable), particle shape (spherical shape is desired), pore size distribution (tight distributions are desirable), surface smoothness, corrosion resistance, temperature stability, and hydrophilicity (hydro-neutral to phobic is desired).
Ceramic proppants dominate sand and resin-coated sand on the critical dimensions of crush strength and hardness. They offer some benefit in terms of maximum achievable particle size, corrosion and temperature capability. Extensive theoretical modeling and practical case experience suggest that conventional ceramic proppants offer compelling benefits relative to sand or resin-coated sand for most formations. Ceramic-driven flow rate and recovery improvements of 20% or more relative to conventional sand solutions are not uncommon.
Ceramic proppants were initially developed for use in deep wells (e.g., those deeper than 7,500 feet) where sand's crush strength is inadequate. In an attempt to expand their addressable market, ceramic proppant manufacturers have introduced products focused on wells of intermediate depth.
Resin-coated sands offer a number of advantages relative to conventional sands. First, resin coated sands exhibit higher crush strength than uncoated sand given that resin-coating disperses load stresses over a wider area. Second, resin-coated sands are “tacky” and thus exhibit reduced “proppant flow-back” relative to conventional sand proppants (e.g. the proppant stays in the formation better). Third, resin coatings typically increase sphericity and roundness thereby reducing flow resistance through the proppant pack.
Ceramics are typically employed in wells of intermediate to deep depth. Shallow wells typically employ sand or no proppant. As will be described in later sections, shallow “water fracs'” represent a potential market roughly equivalent to the current ceramic market in terms of ceramic market size.
The family of non oxide based ceramic materials, specifically the carbides and nitrides of metallic materials, display exceptional mechanical, thermal and chemical properties all of which in combination would be ideal candidates for a proppant system. Although, they display very high intrinsic failure strengths, hardnesses, and fracture toughnesses, their apparent properties are highly dependent upon the microstructure of the ceramic material that develops during the sintering stage. Significant research has been conducted in the sintering of the carbide and nitride class of materials, the most important of which is the use of a glass forming liquid phase sintering aid to assist with the densification of the system. Although, the liquid phase sintering approach assists with the densification, the properties of such a system are less than optimal and fail to reach the intrinsic properties that these materials are capable of, due primarily to the effects of a relatively weak phase existing at the grain boundaries of the ceramic material. In addition, with the liquid phase sintering approach, a high level of shrinkage occurs during sintering. The degree of shrinkage is dependent upon a number of parameters, the most critical of which is particle size. Typically the degree of shrinkage can approach 20% or higher.
Another approach to improve the sintering and consequently the properties of such ceramic systems has been with a reaction mechanism that forms the appropriate carbide and/or nitride phase directly from the metallic phase. In this method, a preform of the appropriate metal is produced, with approximately 25-30% percent residual porosity. The component is then subjected to thermal treatments under the appropriate atmosphere to induce the formation of the carbide or nitride phase. During the formation of the carbide or nitride phase, a volume increase occurs, which serves to close the residual porosity and yield a highly dense ceramic body that is more or less pore free. By carefully controlling the initial porosity of the preform, the volume expansion associated with the formation of the carbide or nitride phase will completely fill all internal porosity and the outer volume of the preform will remain unchanged. This process is termed net shape forming.